Mass spectrometers are widely used to separate and analyse ions on the basis of their mass to charge ratio (m/z). For certain types of mass spectrometer which acquire data in the form of a transient, for example by detection of an induced oscillating image current, the use of Fourier transforms (FT) is a well known and established data processing technique enabling high resolution mass spectra to be obtained from mass spectrometers. Various other transforms may be used but Fourier transforms are by far the most widely used due to their relative high speed and simplicity. Description of the FT technique can be found, for example, in Marshall, A. G. & Verdun, F. R., Fourier Transforms in NMR, Optical and Mass Spectrometry; A User's Handbook, Elsevier, 1990. Examples of FT mass spectrometers include Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometers and ion traps that measure the frequency of ion oscillation induced by an electrostatic potential that varies harmonically in one direction such as the Orbitrap™ mass spectrometer from Thermo Fisher Scientific (herein referred to as harmonic potential-FTMS).
In the aforesaid types of FT mass spectrometer the ions being analysed are urged to undergo oscillatory motion within the spectrometer which induces a correspondingly oscillatory image charge in neighbouring detection electrodes which enables detection of the ions. The oscillatory motion may be of various forms including, for example, circular oscillatory motion in the case of FT-ICR and axial oscillatory motion whilst orbiting about a central electrode in the case of certain harmonic potential-FTMS such as an Orbitrap™ MS. The oscillatory image charge in turn induces an oscillatory image current in circuitry connected to the detection electrodes, which is then typically amplified, digitised and received by a processor such as a computer as a transient (i.e. a signal in the time domain). The oscillating ions induce oscillatory image current at frequencies which are related to the mass-to-charge (m/z) values of the ions. Each ion of a given mass to charge (m/z) value will oscillate at a corresponding given frequency such that it contributes a signal to the transient which is generally in the form of a sine-shaped wave at the given frequency. The total detected image current of the transient is then the resultant sum of the image currents at all the frequencies present (i.e. a sum of sine waves signals). Fourier transformation of the transient yields the oscillation frequencies associated with the particular detected oscillating ions and from the frequencies the m/z values of the ions can be determined (i.e. the mass spectrum) by known equations.
For all types of mass spectrometry there is a desire to improve the resolving power of the instrument. For example, a method to counter peak broadening due to the detector in time-of-flight MS is disclosed in US 2003/0218129 A1.
Various methods exist to increase the resolving power of mass spectrometry employing image current detection of oscillating ions such as Fourier transform mass spectrometry (FTMS). Probably the best known method is to increase the detection time. If the transient signal itself has sufficient duration a doubling of the detection time doubles the resolving power of the system. However, FTMS requires relatively long detection times to achieve high resolving powers. This method of increasing the resolving power by increasing detection time has potential drawbacks, such as signal decay over time (e.g. due to de-phasing, or ion loss by collisions) which limits the maximum achievable resolution. The signal may also deteriorate over time, e.g. due to frequency drift, further limiting the maximum achievable resolution or resolving power. Some of these effects are now described in more detail.
An increase of resolving power of m/z analysis in FTMS by extension of the detection time requires acquiring longer transients of image current induced on the detection electrodes by coherent oscillations of ion packets. However, the coherency of an ion packet or even its very existence cannot always be supported for a long time. This is especially true for analysis of intact proteins which can suffer rapid decay in FT mass spectrometers because of collisions with residual gas and sometimes metastable fragmentation. The higher the mass of proteins, the more resolving power is required to resolve its C-13 isotopes and frequently the faster is the decay of such proteins in FTMS (primarily due to collisions). At present this limits the mass of the highest isotopically resolvable protein to approx. 110 kDa for FT-ICR and 70 kDa for an Orbitrap MS. At the same time, increasing pharmaceutical importance of heavier proteins (such as antibodies with MW around 150 kDa) emphasizes the need for more comprehensive and accurate analysis of these proteins and their modifications. The ability to resolve isotopically these forms would greatly increase reliability of identification of such modifications. It is thus desirable to further increase the resolving power for a given detection or acquisition time.
However, there exist obstacles to the improvement of resolving power for a given detection or acquisition time. Technical solutions like, e.g., increase of the magnetic field in FT-ICR-MS or changes to the field geometry and voltages of an Orbitrap™ MS, may be difficult or prohibitively expensive.
For a given signal the resolution or resolving power can be improved by various well known mathematical methods, such as: (i) Linear prediction, Autocorrelation and/or maximum entropy methods [see Marshall, A. G. & Verdun, F. R., Fourier Transforms in NMR, Optical and Mass Spectrometry; A User's Handbook, Elsevier, 1990, chapter 6], which all require major software development and have not yet been routinely used in FTMS due to their high computational expense when compared with the expected resolution gains; (ii) display in absorption mode [see Marshall, A. G. & Verdun, F. R., Fourier Transforms in NMR, Optical and Mass Spectrometry; A User's Handbook, Elsevier, 1990, chapter 2.4.2]; and (iii) FSD (Fourier self deconvolution) if resolution is limited by other factors than detection time, and Peak fitting, i.e. involving fitting multiple model peaks to one real peak, both of which usually require an a priori knowledge of the peak shape. Moreover, the aforementioned mathematical methods typically promise only moderate increases of resolution or resolving power (e.g. about 2 to 3 times) although most of the aforementioned techniques can be utilised together with the present invention as will be apparent below.
A totally different technical approach is to detect the flight time of ions ejected from an FT-ICR cell after resonant excitation and infer the exact mass from small differences in the flight time induced by the cyclotron resonance [see Becker, S.; Bollen, G.; Kern, F.; Kluge, H.-J.; Moore, R. B.; Savard, G.; Schweikhard, L. & Stolzenberg, H.: Mass Measurements of Very High Accuracy by Time-Of-Flight Ion Cyclotron Resonance of Ions Injected into a Penning Trap; International Journal of Mass Spectrometry and Ion Processes, 1990, 99, 53-77]. This indirect measurement by ion ejection is currently only proven for FT-ICR and used in heavy isotope research because it is very fast (no necessity for transient detection).
Accordingly, there remains a need to improve the resolving power in mass spectrometry, especially for mass spectrometry using image current detection. In view of the above background, the present invention has been made.